The coalition government has launched a cynical attack on people with mental health problems, says Clare Allan. Photograph: Andrew Parsons/EPA

The government's plan to force everyone receiving disability living allowance (DLA) to undergo a medical assessment is deeply misguided. If allowed to go ahead, the consequences could be catastrophic for people with mental health problems.

Looked at alongside George Osborne's assault on incapacity benefit, capping of housing benefit (it was the Conservatives whose abolition of rent controls led to soaring rents in the first place), and the decision to link benefit rates to the consumer prices index instead of the retail prices index, it is hard to escape the conclusion that these changes constitute a cynical attempt on the part of the coalition government to manipulate public prejudice towards those least able to speak up for themselves.

Around half of all people claiming benefits because of illness have mental health problems. The cost of providing for them is considerable. And essential. Any decent society should be urgently concerned with ensuring those affected receive the support they need to lead full and rewarding lives.

In some cases, this will involve working with people to help them develop the skills and confidence they need to return to the workplace or to enter it for the first time. Where such schemes exist, they are largely confined to the voluntary sector, often run by people with personal experience of the challenges involved, and always oversubscribed.

It is a fallacy, and a deeply offensive one, that people with mental health problems do not want to work. According to a report published in 2004 by the Social Exclusion Unit, mentally ill people have the highest "want to work" rate of any disability group. It also found that fewer than four in 10 employers would consider offering a job to someone with a history of mental illness.

Providing people with the help they need, and addressing the stigma that holds them back, is no easy task. It takes time, thought and significant financial investment. How much easier to harness the stigma and turn it to political advantage! The welfare system is overrun with work-shy benefit cheats. The "allegedly disabled" (as Jeremy Paxman put it recently on Newsnight) are living it up on taxpayers' money, claiming benefits to which they are not entitled. We must reassess everyone, weed out the scroungers! What objection could anyone have? Unless they're cheating.

Let's think about that. DLA exists to help those with serious long-term health problems pay for the additional support they need as a result of their condition. It is not means tested. Many working people receive DLA. It is often DLA that enables them to work. It isn't easy to get. Claimants must complete a lengthy and personal form covering everything from toileting needs to suicidal urges. A statement is required from the claimant's doctor as well as someone who knows them well – often, in the case of psychiatric patients, their social worker or community psychiatric nurse. Fewer than half of all claims are successful.

Now, with the openly stated aim of reducing the benefit bill, the government will employ its own assessors to decide who is worthy of help. NHS psychiatrists, who have the right to deny people liberty and to medicate them against their will, are not to be trusted when it comes to providing an accurate statement of an individual's support needs.

As history has shown, the intangibility of mental illness, the lack of a definitive test, provides opportunities for exploitation. Unmarried mothers, troublesome offspring, homosexuals, all have been detained in psychiatric hospitals. "Proof" of mental health or ill health is impossible to provide. An assessment inevitably involves a subjective judgment on the part of the assessor. And when the assessor has an outside agenda – reducing the benefits bill, say – the lack of objective proof leaves mentally ill people vulnerable.

The awkward truth is that mental health problems are real. They cannot be bullied or wished away. If support is removed, this doesn't mean support is not required. It means that as a society we have chosen to look away. We must not allow this to happen. The fight is on.